As well known, in the paper converting industry a variety of types is used of machines and of processes for making paper tissues, paper towels and similar articles from stacks of folded, overlapped or interfolded sheets of a certain height.
The stacks are obtained by folding the sheets and simply overlapping them on one another, or by interfolding them, i.e. at each fold a wing of a previous sheet and a wing of a next sheet of the stack engage with each other. This way, when drawing a sheet from a package, in use, a wing of a next sheet of the stack is dragged and pops out from the package, with subsequent practical utilization for certain types of users. Among possible interfolding ways the L-type, with 2 folds (single fold), or the Z or W types, respectively with 3 and 4 folds (multi fold), are known.
After a step of separation of a pack from the stack, once achieved a measured height of the pack, a step follows of conveying the packs or logs for further treatment and packaging.
The stacks of folded or interfolded sheets from which a log is formed have a length that depends on the width of the starting web, normally about 2 meters and more.
For example, in case of logs of interfolded products exiting from the head of an interfolding machine, according to the type of interfolded product and then to the final use, said log can be conveyed to a transversal cutting machine forming small packages of predetermined length and then forwarding them to a binder that places them into customized wrappers tailored for the users. With this solution, normally, the so-called “facial tissue” products are packaged.
Alternatively, the products can be distributed as banded packs or clips, as is the case of “hand towels”, which can be placed into special fixed distributors with standard shape. In this case, the whole log of interfolded product, normally, is first compressed and then banded by a web that covers it around, creating a tubular wrapper. The banded log is then cut off into single small packages or clips that are already banded by a corresponding cut tubular wrapper portion.
In the latter case, for carrying out the banding step, banding machines are used that are located downstream of the interfolding machine. In particular, the log of interfolded sheets that have been previously produced and that exit from the interfolding machine can be carried directly into the banding machine by means of an appropriate conveying system, such as a conveyor belt.
A diagrammatical view of the operation of a known banding machine is described in FIGS. 1-4, respectively a longitudinal sectional view and cross sectional views of three successive operation steps with respect to the log. With reference to FIG. 1, a log 1 of interfolded or folded material is carried by a conveyor belt 2 into a banding machine of the prior art. When entering into the banding machine, log 1 follows a path set between two overlapping conveyor belts 3 and 4 that gradually decrease their relative distance in the vertical direction in order to compress log 1, eventually reaching a compressed configuration 1′ between two overlapped conveyor belts 5 and 6. Once compressed, the logs are banded by means of a wrapper 11 (FIGS. 2-4) that is cut from a web, by a pusher 7 that is operated by an actuator 8 and pushes log 1 into a channel 10 of the machine in order to carry out the banding step. More precisely, compressed log 1′ is pushed into channel 10, and a cut sheet of web 11 is arranged stretched as an obstacle for the log along all the length of the machine, in a passage 12. In a way not shown, the cut sheet of web 11 is automatically unwound from a roll and cut at a predetermined length. Therefore, by the action of a pusher 7, operated by a cylinder piston 8, which pushes the log against the stretched sheet beyond passage 12, a kind of self-packaging is carried out, i.e. the log is wound by the sheet for of its side surface. Then, after the withdrawal of pusher 7, a system of plates 9 transversal to channel 10 is actuated, in order to align to log 1′ the remaining wings of sheet 11, on one of which a predetermined amount of glue was previously spread, so that they are closed in turn on the free side of log 1′. Then, further compressed and banded logs are pushed progressively forward against log 1′, which moves in channel 10 up to an exit port where it is withdrawn in a way not shown.
In other known systems, instead, there is a movable upper support that compresses the log from the above on a conveyor belt, a predetermined position has been reached, thus reducing the volume and the height of the log before the banding step. For example, a machine of this type is described in EP1636093.
During the forward and back strokes of the pusher, the conveyor belt must stay, for allowing the return stroke of the pusher. For example, in case of a one meter travel of the pusher, i.e. about half meter of forward stroke and half meter of back stroke, a certain time is necessary, which causes a reduction of productivity, for allowing the back stroke of the pusher.
It is therefore desirable to minimize the waiting time to improve productivity.
Concerning the log compression step, a similar system is described in U.S. Pat. No. 6,865,861. In this case, two conveyor belts decrease their relative distance (see also FIG. 1), to improve productivity with respect to the case of a single conveyor belt and pusher. In fact, while in the first case the log is compressed when entering the machine, in the latter case the log must enter completely in the banding machine for pressing it, increasing the waiting time.
In U.S. Pat. No. 6,865,861 there is thus a need to feed the web that is used for wrapping the log, causing the web to be frictionally dragged by the log same. This has the drawback that the speed, and then the productivity, must be low, and the compression of the log cannot be excessive, to avoid break of the banding machine web, or to avoid difficulty of introduction and of drag of the web between the conveyor belts and the log.